The nudibranch family Dironidae. 521 
and slightly flattened antero-posteriorly. In Dirona pieta they are 
roughened with tubercules, notably on the inner surface where 
these elevations form a longitudinal ridge with a series of irregular 
branchings extending laterally from it (Pl. 30 Fig. 1). These eleva- 
tions contain blood vessels close beneath their surface, and are pro- 
bably of respiratory significance. In Dirona albolineata the cerata 
are smooth throughout. Each cera is made up of a loose connective 
tissue framework, covered externally by a continuation of the low, 
columnar epithelium which clothes the general surface of the body. 
Two longitudinal blood vessels, an external and an internal, con- 
nected with each other by abundant lateral branches, which course 
near the surface, are found in each cera. The outermost of these 
connects with a large, longitudinal sinus at the base of the cerata 
rows, and receives blood through it from the general body sinuses. 
The inner vessel, receiving the numerous superficial branches con- 
tained in the surface ridges, connects with an inner dorsal, longi- 
tudinal trunk passing directly to the auricle At the base of the 
cera each of these trunks is provided with a well developed sphincter 
muscle by which the lumen may be closed in case of the accidental 
loss of such an exposed and delicate structure. Through these 
muscles the relative distension of the cera with blood may also be 
controlled. Thus the cerata are definitely respiratory in function, 
sharing this, however, with the general body surface. Each cera is 
provided with muscle fibres radiating into it from its base and 
attached at their proximal ends to the adjacent body wall. Other 
fibres pass in various directions through the cera and accomplish 
its different movements. An abundant nerve supply is furnished by 
branches of nerves from the pleural ganglia, which form com- 
plicated anastomoses within the organ, including in their course a 
large number of small peripheral ganglia. The terminal fibres are 
distributed to the integument and to the muscle fibres. 
In the posterior half of the body there extends forward, just 
below the bases of the cerata, along the upper margin of the side 
of the animal, a narrow, glandular band, which, in Dirona pieta, en- 
circles the posterior end of the dorsum, extending forward on each 
side of the animal to about the same distance. In Dirona albolineata 
this band appears to be limited to the right side, or at least is 
much better developed there. It extends forward to the region of 
the reproductive openings, and is made up of a densely packed mass 
